Handle unexpected clear events call

Currently we don't check if we have access to events at the time of
calling clear_events for an instance. This could result in operations
failing (like deleting an instance) for the user.

Add a check and a log message that this shouldn't happen (but it does).

Change-Id: I29852662cbfe38f37f9d5154147e6b17b7b8b454
Closes-Bug: 1454149
(cherry picked from commit bb4511b)

nova/compute/manager.py

@@ -554,6 +554,10 @@ def clear_events_for_instance(self, instance):
         """
         @utils.synchronized(self._lock_name(instance))
         def _clear_events():
+            if self._events is None:
+                LOG.debug('Unexpected attempt to clear events during shutdown',
+                          instance=instance)
+                return dict()
             return self._events.pop(instance.uuid, {})
         return _clear_events()
 

nova/tests/unit/compute/test_compute_mgr.py

@@ -1756,6 +1756,12 @@ def test_pop_events_fails_gracefully(self):
         self.assertIsNone(
             self.compute.instance_events.pop_instance_event(inst, event))
 
+    def test_clear_events_fails_gracefully(self):
+        inst = objects.Instance(uuid='uuid')
+        self.compute.instance_events._events = None
+        self.assertEqual(
+            self.compute.instance_events.clear_events_for_instance(inst), {})
+
     def test_retry_reboot_pending_soft(self):
         instance = objects.Instance(self.context)
         instance.uuid = 'foo'